Thank you for joining the thread, @Ina Farmer.

Allow me to share some information about the error message you're getting.

The ability to move the customer to another sub-customer or unmark the sub-customer isn't available if the sub-customers transactions are associated with the parent customer through the Bill with parent option. You can only achieve your goal if the customer and sub-customer have individual entries in the system.

That said, you need to delete all the payments and invoices associated with the customer that you're trying to modify. This is to ensure that your records are accurate and to prevent issues in the future.

 I'm adding a link that you can check out about managing your customer's information in QuickBooks Online from adding to restoring them, feel free to read the details from here during your free time: Manage your customer list.

Please know that I'm only a post away if you have any other issues or concerns. I'll be more than happy to help. Take care always!